1. Field of the Invention
The present invention relates to an aluminum material in which an anodic oxidation film is formed on a surface of an Al alloy, and in particular to an Al material suitable for a vacuum container in an apparatus for producing a semiconductor or a liquid crystal, as a material excellent in thermal crack resistance and corrosion resistance in high temperature and corrosion environment.
2. Discussion of the Related Art
Apparatuses for producing semiconductors, semiconductor devices, liquid crystals, and liquid crystal displays, such as a chemical or physical vacuum evaporation apparatus for CVD or PVD, or a dry etching apparatus, are composed of main elements such as a heater block, a chamber, a liner, a vacuum chuck, an electrostatic chuck, a clamper, a bellows, a bellows cover, susceptor, a gas diffusion plate, electrodes and the like. A corrosive gas containing a halogen element such as Cl, F or Br, and an element such as O, N, H, B, S or C are introduced as a reactant gas into these apparatuses and, in consequence, corrosion resistance against the corrosive gas (gas corrosion resistance) is required in these main elements. In addition to the corrosive gas, halogen-based plasma is also generated, and thus corrosion resistance against the plasma is also required.
Stainless steel has heretofore been used as a preferred material. However, because of recent demands for higher efficiency and lighter weight of apparatuses for producing semiconductors, semiconductor devices, liquid crystals, and liquid crystal displays, the following problems arise in stainless steel members: their heat conductivity is insufficient, which requires much time for starting the apparatuses; the weight of the stainless steel elements is large, thus making the whole weight of the apparatus larger; and heavy metals, such as Ni and Cr, which are contained in the stainless steel are released in the producing process, which lowers the quality of semiconductor or liquid crystal products.
For this reason, the use of aluminum (Al) alloys, which are light and have high heat conductivity, is rapidly increasing in the place of stainless steel. Among the Al alloys, the following alloys are widely used: JIS 3003 Al alloy containing Mn: 1.0-1.5%, Cu: 0.05-0.20% and the like, JIS 5052 Al alloy containing Mg: 2.2-2.8%, Cr: 0.15-0.35%, and the like; and JIS 6061 Al alloy containing Cu: 0.15-0.40%, Mg: 0.8-1.2%, Cr: 0.04-0.35%, and the like. However, the surface of these alloys does not have excellent corrosion resistance against the corrosive gas and the plasma. Thus, in order to use such Al alloys as a material for a vacuum container in an apparatus for producing semiconductors, semiconductor devices, liquid crystals, and liquid crystal displays, it is essential to improve the corrosion resistance against such gas and plasma. In order to improve the corrosion resistance against the gas and the plasma, the most effective means is typically that some surface-treatment is applied to the surface of the Al alloys.
Thus, Japanese Patent Application Publication (JP-B-) No. 5-53870 proposes an invention for forming an anodic oxidation (Al.sub.2 O.sub.3) film having an excellent corrosion resistance on the surface of the Al alloys to improve the corrosion resistance of vacuum chamber elements and the like against gas and plasma. However, such anodic oxidation films have very different corrosion resistance against the gas and the plasma depending upon the quality of the films. Consequently, the demand for corrosion resistance cannot be satisfied in some environments in apparatuses for producing a semiconductor.
Therefore, various inventions are proposed for making the quality of the anodic oxidation film higher in order to improve the corrosion resistance of the Al alloy as members in apparatuses for producing semiconductors and the like. For example, Japanese Patent Application Laid-Open (JP-A) No. 8-144088 proposes an invention in which an ending voltage for anodic oxidation is made higher than an initial voltage when the anodic oxidation film is formed. JP-A-8-144089 proposes an invention in which an anodic oxidation treatment is carried out in a solution containing sulfuric ion or phosphoric ion to make the level of concaves in the surface of the anodic oxidation film fall within a specified range. Furthermore, JP-A-No. 8-260196 propose inventions in which a porous type of anodic oxidation treatment is carried out and then a non-porous type of anodic oxidation treatment is carried out.
In all of the aforementioned conventional methods that relate to anodic oxidation, as shown in FIG. 4, while concave portions called pores 3 are formed on the surface of an Al alloy substrate from the beginning of the electrolysis, basically, an anodic oxidation film comprising a porous layer 4 composed of cells 2 which will grow in the depth direction of the Al alloy 1 and a barrier layer 5 having no pores is formed. Since the barrier layer 5 having no pores does not have gas permeability, gas and plasma are prevented from contacting the Al alloy 1 .
Similarly, JP-A-No. 8-193295 and the like propose inventions in which the pore diameter and the cell diameter at the side of the surface of the porous layer 4 are reduced as much as possible, in order to improve plasma corrosion resistance of the anodic oxidation film having this double structure. Indeed, the anodic oxidation film having the porous layer 4 and the barrier layer 5 with no pores, wherein the pore diameter and the cell diameter at the surface side of the porous layer 4 are reduced as much as possible, is excellent in corrosion resistance against the gas and the plasma.
However, the conditions for producing semiconductors, semiconductor devices, liquid crystals, and liquid crystal displays are becoming very strict because of recent demands for increasing the efficiency and size of the apparatus. Concerning the gas and plasma conditions, higher concentration, high density and higher temperature are required. Therefore, constituent members of a reacting container (chamber) and the members inside it need to have corrosion resistance against a corrosive gas containing a halogen element such as Cl, F or Br, or an element such as O, N, H, B, S or C, or plasma, and such demand is becoming increasingly stricter in recent years.
As a consequence, the anodic oxidation films obtained by the aforementioned anodic oxidation treatments cannot satisfy the stricter demands for the corrosion resistance against the gas and the plasma.
In addition, in recent years the demand for heat resistance of materials for apparatuses for producing semiconductors is also becoming increasingly strict. As described above, depending on the process conditions for producing semiconductors, the members for apparatuses for producing the semiconductors are subjected to heat cycles at high temperature many times during use. In the anodic oxidation film obtained by the aforementioned anodic oxidation treatment, therefore, cracks arise in these high temperature heat cycles, and, in the corrosive environment associated with the gas and the plasma, corrosive components invade the cracks in the anodic oxidation film, resulting in a problem of corroding the aluminum alloy base material. Therefore, in order to satisfy the demand for heat resistance of materials for apparatuses for producing semiconductors, semiconductor devices, liquid crystals, and liquid crystal displays, it is necessary to prevent the generation of cracks in the anodic oxidation film in high temperature thermal cycles, that is, improve the thermal crack resistance.